Since the 1960s, there have been numerous, high profile incidents of ‘spontaneous” failure of toughened glass in building facades. In some cases, glass has been known to simply ‘pop’ out of high-rise building windows and fall onto the streets below  Why does this happen? The cause lies within the makeup of the glass itself.

Nickel sulphide is a rare and unintended inclusion in the production of glass panels. However, the presence of nickel sulphide can be a problematic one. This is due to what is technically called, ‘delayed transformation’.

For those interested in technical detail, these follow:

Nickel sulphide crystals can take one of two forms. At high temperatures, a dense crystal is present. At lower temperatures, a less dense crystal is present. Provided the transition from high to low temperature (ie. cooling) is gradual, the crystal can move between forms without problem.

For example, in ordinary annealed glass, the crystals do not cause problems because the cooling process occurs slowly during manufacture.

However, during the production of glass for use in high rise buildings, a very fast cooling process is used. This process in necessary to create the toughened glass that is capable of withstanding exposure to strong winds.

The problem begins when the glass is cooled rapidly during the toughening process. This rapid cooling process often fails to cool the nickel sulphide crystals. So the nickel sulphide remains trapped in its high temperature form. However, the story doesn’t necessarily stop there. Sometimes, years later, the crystals may spontaneously start to transform and result in the glass breaking, causing the windows to shatter onto the streets below.

This problem still presents itself in glass manufactured today. If a significant amount of toughened glass is used, there is likely to be nickel sulphide inclusions. Statistically speaking, around two out of every 1000 toughened glass panels that are produced are likely to contain nickel sulphide inclusions.

The solution is not particularly feasible at this point in time. Existing detection methods are expensive and time consuming. The only way to detect and remove the potential for nickel sulphide is to examine each pane of glass individually. Each image must then be analysed for the detection of inclusions. Once these inclusions are detected the panes of glass are removed and the process of heat soaking begins. Heat soaking is a process first used in 1982 which heats up the glass pane enough to cause the inclusions to change phase altogether, thereby removing them. At this stage there is a 95% conversion rate for removing the inclusions via heat soaking. 

However, this is not the same as a 100% success rate, and it is indeed a time consuming and expensive process. While there are new developments within the industry that one day may overcome this problem, the shattering truth remains that when toughened glass is used, nickel sulphide inclusions are more likely than not to present themselves.